The present invention relates to an optical system and, more particularly, to a decentered optical system having power, which comprises a decentered reflecting surface.
There has heretofore been known a compact reflecting decentered optical system as disclosed in Japanese Pat. Appln. Laid-Open (KOKAI) No. 59-84201. This is an invention of a one-dimensional light-receiving lens comprising a cylindrical reflecting surface; therefore, two-dimensional imaging cannot be effected with the conventional optical system. Japanese Pat. Appln. Laid-Open (KOKAI) No. 62-144127 discloses an optical system wherein the identical cylindrical surface is used twice to effect reflection in order to reduce spherical aberration in the above-mentioned invention. Japanese Pat. Appln. Laid-Open (KOKAI) No. 62-205547 discloses the use of an aspherical reflecting surface as a reflecting surface, but makes no mention of the configuration of the reflecting surface.
U.S. Pat. Nos. 3,810,221 and 3,836,931 both disclose an example in which a rotationally symmetric aspherical mirror and a lens system having a surface which has only one plane of symmetry are used to constitute a finder optical system of a reflex camera. In this example, however, the surface having only one plane of symmetry is utilized for the purpose of correcting the tilt of a virtual image for observation.
Japanese Pat. Appln. Laid-Open (KOKAI) No. 1-257834 (U.S. Pat. No. 5,274,406) discloses an example in which a surface having only one plane of symmetry is used for a reflecting mirror to correct image distortion in a back projection type television. In this example, however, a projection lens system is used for projection onto a screen, and the surface having only one plane of symmetry is used for correction of image distortion.
Japanese Pat. Appln. Laid-Open (KOKAI) No. 7-333551 discloses an example of a back-coated mirror type decentered optical system using an anamorphic surface and a toric surface as an observation optical system. However, the decentered optical system is not sufficiently corrected for aberrations, including image distortion.
None of the above-described prior art use a surface having only one plane of symmetry as a back-coated mirror to form a turn-back optical path.
In the conventional rotationally symmetric optical systems, a transmitting rotationally symmetric lens having refracting power is assigned to exert the required refracting power. Therefore, many constituent elements are needed for aberration correction. In the conventional decentered optical systems, however, an imaged figure or the like is undesirably distorted and the correct shape cannot be recorded unless aberrations of the formed image are favorably corrected and, particularly, rotationally asymmetric distortion is favorably corrected.
In a rotationally symmetric optical system comprising a refracting lens which is formed from a surface rotationally symmetric about an optical axis, a straight-line optical path is formed. Therefore, the whole optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large-sized apparatus.
In view of the problems associated with the prior art, an object of the present invention is to provide a compact optical system capable of providing a clear image of minimal distortion even at a wide field angle.